$ D = \left[\begin{array}{rr}4 & -1 \\ 0 & 4 \\ -1 & 0 \\ 3 & 1\end{array}\right]$ What are the dimensions of matrix $ D$ ?
Solution: The first dimension is the number of rows in the matrix. How many rows does $ D$ have? The second dimension is the number of columns in the matrix. How many columns does $ D$ have? So, $ D$ is a 4 $\times$ 2 matrix.